Radiation detection systems generally employ a radiation detector such as a germanium or scintillation detector to detect radiation from a radiation source such as alpha or gamma rays. The detection of such energy results in a charge pulse whose amplitude is proportional to the energy of the incident radiation. The charge pulse is converted to a voltage pulse by a charge sensitive preamplifier. The voltage pulse is filtered with an analog or digital filter to improve the accuracy and precision of the measurement and the amplitude of the resulting pulse is measured. Usually the measured value is histogrammed to form a spectrum which indicates the number of pulses of a particular amplitude which have been processed by the system as a function of the amplitude of the pulse.
Since the measured amplitude of the pulse is proportional to the energy of the incident radiation, the spectrum may also be interpreted to indicate the number of alpha or gamma-rays of a particular energy which have been processed by the system as a function of the energy of the incident radiation. To make this conversion between measured amplitude and energy, a radiation source of known characteristics is analyzed by the detection system. Since the construction of the radiation source is known features in the spectrum can be used to calibrate the detection system. For example, if a CO-60 source is analyzed by a detection system, two peaks will appear in the spectrum. The energy of the higher peak is known to be 1332.5 keV and the energy of the lower peak is known to be 1173.237 keV. With this information, a mapping can be made between the measured amplitude of the pulse and the energy of the incident radiation.
Once this calibration has been made, it is desirable for the system to be sufficiently stable such that the calibration remains valid for long periods of time. Ideally the calibration would never change. Unfortunately, in real world systems the measured amplitude changes with varying conditions due to changes in the environment and also due to changing intensity of the radiation source. Such changes can be either gain errors or offset errors.
A circuit which has been widely used in analog systems to eliminate the offset errors is the gated-base line restorer. A gated-base line restorer is a time-variant circuit which averages the output from the filter when no pulses are being processed by the filter and subtracts the resulting average from the output of the filter. When pulses are being processed by the filter, the average holds its current value. By using this gated approach, the shape of the output pulse is not disturbed by the restorer, yet any offset errors in the system are effectively removed.
In a system which has a digital filter, the shifts in offset are reduced due to the ideal nature of digital circuits; however shifts from the preamplifier and sampling circuit would still cause offset shifts at the output of the digital filter.
Georgiev and Gast, IEEE Trans. on Nucl. Sci. Vol. 40, No. 4 (August 1993), have pointed out one technique for performing this function digitally in which a moving average is used to determine the amount of offset error to remove. In this circuit, all base line points are weighted equally regardless of how old the points are.